Accumulator injection systems for internal combustion engines, which are also referred to as common rail injection systems, typically have two final control elements to regulate operating parameters of a fuel to be injected. The metering unit is provided as the first final control element, using which a quantity regulation of the fuel takes place. Pressure regulation of the fuel takes place using the pressure regulating valve as the second final control element.
During a pressure regulating operation, the pressure regulating valve is operated by being solely controlled via the metering unit. A control of the pressure regulating valve is designed in the case of the pressure regulating operation in such a way that the pressure regulating valve remains closed with all relevant tolerance sources being taken into account.
If this is not ensured, a permanent leak may result at the pressure regulating valve, due to which impermissible heating of the injection system and/or fuel as well as an increase in the fuel consumption may result. The leak may be detected by monitoring functions and a limp-home operation may be initiated thereupon.
On the other hand, a required tolerance allowance at the pressure regulating valve may have the result that injection systems are operated using an excessively high opening pressure at the pressure regulating valve depending on the position tolerance. However, in case of an error, for example, a metering unit which is stuck open, this may prove to be disadvantageous, because pressures far above the nominal pressure or setpoint pressure may thus result as a possible reference value for the control pressure of the injection system. The increased pressures arising in such a case of error must not result in a failure of the injection system.
Excessively high pressures may cause ruptures of lines of the injection system and therefore an escape of fuel into the engine compartment. Furthermore, the durability of components in the injection system may be reduced, since their failure may be induced. In addition, a loss of the limp-home capability may occur, using which a vehicle is typically moved out of a danger zone.
In consideration of a typical error detection and response time, it is necessary under these given conditions to design components of the injection system to be appropriately complex and robust.
Furthermore, so-called adaptation functions for determining a characteristic of the pressure regulating valve are provided for a pressure regulating valve, such a characteristic typically being a characteristic curve of the pressure regulating valve.
In these adaptation functions for a pressure regulating valve which is capable of adaptation and/or learning, learning only takes place during pressure-regulated operation. The adaptation takes place in the event of nearly arbitrary and thus only slightly controllable boundary conditions of the pressure and a flow rate of the fuel through the pressure regulating valve. A flow rate through the pressure regulating valve to be provided for the adaptation is substantially greater than 0 L/h, whereby a tolerance definition for the opening pressure of the pressure regulating valve worsens. The highest pressure at which learning may take place using these adaptation functions is much less than a pressure limit of the injection system. Proceeding from a learned pressure, the tolerance definition worsens with increasing distance therefrom.
These adaptation functions offer the possibility of reducing the flow rate during the adaptation proceeding from the quantity of fuel which results from a full delivery of the pump. However, further tolerance sources arise in this way, which may worsen an overall result of the adaptation.
German Application No. DE 10 2004 006 694 A1 describes, inter alia, a method for operating an internal combustion engine having an injection system, in particular for a motor vehicle. In the injection system, fuel is conveyed by a metering unit and a high-pressure pump into a fuel accumulator. The pressure in the fuel accumulator is registered and regulated by the control unit by activating the metering unit. In this system, in order to also take into consideration possible manufacturing tolerances of individual metering units in the regulation of the pressure in the fuel accumulator and to make the regulation more precise in this way, it is proposed that an individual characteristic curve be ascertained for the particular instantaneously used metering unit and be taken into consideration in the pressure regulation. Therefore, an individual characteristic curve, typically for a pump, is learned using the described method.
In a fuel system for an internal combustion engine described in German Application No. DE 10 2004 059 330 A1, the fuel is conveyed by a high-pressure fuel pump into the fuel pressure accumulator, from which the fuel reaches at least one combustion chamber of the internal combustion engine via at least one injector. A value for the actual pressure in the fuel pressure accumulator is provided. A pressure regulating unit, using which fuel may be discharged from the fuel pressure accumulator, is pilot controlled using a pilot control signal, which is ascertained in consideration of a setpoint pressure in the fuel pressure accumulator. It is proposed that a value for a fuel quantity flowing through the pressure regulating unit be taken into consideration upon the ascertainment of the pilot control signal. A consideration of a flow rate through a pressure regulating valve as the foundation for the mentioned adaptation functions is therefore described here.
A method for operating a fuel injection system, in particular for a motor vehicle, is described in German Application No. DE 10 2004 049 812 A1. The fuel injection system has a fuel accumulator, to which fuel may be supplied via a metering unit. In the method, an actual pressure in the fuel accumulator is influenced, inter alia, by a current regulator. A pilot control value is generated by a pilot control characteristic map, using which production-related deviations of components of the fuel injection system are compensated for. Therefore, an adaptation method based on a characteristic map to compensate for manufacturing tolerances in the case of regulation via the metering unit is described here. This method may also be used for the mentioned adaptation functions due to the use of an integral control element.
German Application No. DE 10 2006 018 164 B3 describes a method and a device for controlling an injection system for an internal combustion engine having a high-pressure pump for conveying fuel into a fuel accumulator, to which an injector is hydraulically coupled. In addition, the injection system includes a volume flow control valve, using which a fuel volume flow through the volume flow control valve into the high-pressure pump is settable, and a pressure control valve. In the method, the pressure control valve is set to a fuel volume flow through the pressure control valve equal to zero. A first control value for the volume flow control valve is determined as a function of a fuel volume flow setpoint value through the volume flow control valve. A deviation of the control value for the volume flow control valve is ascertained as a function of a difference between a setpoint value and an actual value of a pressure in the fuel accumulator. A second control value for the volume flow control valve is determined as a function of a reference fuel volume flow through the pressure control valve. The reference fuel volume flow is sufficiently great that the pressure control valve allows setting of the pressure in the fuel accumulator.
A method for the adaptation of a pilot controller in a pressure regulating unit for a common rail injection system of an internal combustion engine is described in German Application No. DE 10 2005 058 966 B3. The regulating unit includes a pilot controller. The common rail injection system has at least one final control element settable by control signals, which each correspond to control values, for influencing the pressure. In the method, a pilot control value is ascertained to generate at least one control signal for the final control element and the instantaneous pressure is regulated to a predefined setpoint pressure while using the pilot control value by generating the at least one control signal and outputting it to the final control element. For the adaptation, the pilot controller is adapted as a function of at least one instantaneous value of an operating parameter of the internal combustion engine and a control value corresponding to the at least one control signal.
A method for controlling a direct injection internal combustion engine is described in German Application No. DE 100 16 900 C2. Accumulator pressure prevailing in a pressure accumulator of a fuel metering system of the internal combustion engine is regulated using an electrically activatable pressure control valve, via which fuel may be guided from the pressure accumulator into a low-pressure area of the fuel metering system to dissipate the accumulator pressure. It is provided that a pilot controller is connected upstream from the regulating unit of the accumulator pressure, an activation signal for the pressure control valve being ascertained within the scope of the pilot control as a function of the flow rate through the pressure control valve and the accumulator pressure, or the accumulator pressure resulting in the pressure accumulator being ascertained as a function of the flow rate through the pressure control valve and an activation signal for the pressure control valve.
An apparatus for advancing the start of injection of an internal combustion engine, for example, in the event of a cold start or low atmospheric pressure, is known from German Application No. DE 34 10 146 A1. This apparatus is controlled by a directional control valve, to which a relieved bypass line having an adjustable flow control valve is connected in parallel, the adjustability allowing a control of the start of adaptation of the pump characteristic curve to the characteristic curve of the internal combustion engine. In addition, a pressure control valve includes an outflow opening, which is controlled by its pressure control piston, the particular controlled flow rate cross section determining an adaptation of the pump characteristic curve to the characteristic curve of the internal combustion engine.